1) Field of the Invention
The present invention is directed towards a process for obtaining zeolites with titanium inserted into the lattice, starting with synthesized or commercial zeolites, with or without titanium. The invention is directed more particularly towards obtaining zeolites with titanium inserted into the lattice, which can be used in catalysts for the controlled oxidation of organic molecules in the presence of hydrogen peroxide and hydroperoxides, improving the insertion of titanium and removing the extra-lattice titanium, thereby favouring an increase in the activity, stability and efficacy of these catalysts.
2) Background Art
Among the synthesized titanium zeolites, the one most extensively studied is titanosilicalite TS-1, of structural type MFI described in U.S. Pat. No. 4,410,501, the catalytic applications of which cover, inter alia, the hydroxylation of aromatic compounds, the epoxidation of olefins, the oxidation of paraffins and alcohols to ketones, and the ammoximation of cyclohexanone with aqueous ammonia. The studies on this solid have clearly shown that the catalytic properties of titanium zeolites are strictly associated with the method of preparation of the catalyst. If non-crystalline titanosilicates or solid titanium oxide are obtained instead of titanosilicalite crystals, the controlled oxidation reactions obtained in their presence are less selective and less effective. Moreover, it is known that the efficacy of the catalysts depends on the level of insertion of the transition metals into the silicon structure of the zeolite.
Titanium zeolites that are catalytically efficient are particularly difficult to prepare since, if the optimum synthesis conditions are not met, the titanium oxide coprecipitates with non-crystalline titanosilicates. The precipitation of these non-porous phases promotes depletion of titanium zeolite and decreases the selectivity and efficacy of the oxidation catalysts since it favours the decomposition of hydrogen peroxide.
In order to avoid this particularly inconveniencing coprecipitation during synthesis, it is preferred to use purified reagents, free of alkaline cations (see French patent application 94/00978 of Jan. 28, 1994) sometimes in the presence of hydrogen peroxide (see patent application WO 94/02245), which considerably increases the overall cost of the synthesis.
Since it is often difficult to control or reproduce these syntheses, methods for the post-synthesis treatment of titanium-free zeolites have been developed in order to insert titanium or any other transition metal into their structure.
Among the various post-synthesis treatments published, it has become apparent that by treating protonic zeolites of MFI, faujasite or BEA type with TiCl.sub.4 vapours, some of the aluminium contained in the structure was extracted in the form of AlCl.sub.3 vapour and replaced by titanium. (B. Kraushaar and J. Van Hoof, Catalysis Letters 1, 1988, 81). However, since all of these vapours are corrosive, this post-synthesis treatment cannot be viably and easily implemented on an industrial scale. In order to circumvent this major drawback, treatments with titanium halides in solution have been studied: J. Kooyman, J. C. Jansen and H. Van Bekkum in Proceedings of 9th International Zeolite Conference, Montreal 1992, Butterworth-Heinemann 1993, volume 1, 505, treated, in particular, MFI zeolites with solutions of TiF.sub.3 in the presence of dilute hydrofluoric acid (HF). Other workers have treated beta-zeolite with solutions of TiF.sub.3 dissolved in dilute hydrochloric acid (see French patent application 2,694,549 (Plee, Nicolas)). The zeolites obtained by these methods do not appear to be sufficiently active as regards oxidation.
Other researchers have preferred to treat aluminium-rich zeolites such as faujasite, LTL, mazzite, MER, MFI and mordenite with an aqueous solution of (NH.sub.4).sub.2 TIF.sub.6, as in patent application WO 85/04854.
However, it has been observed that the amount of aluminium present in the structure still remained large, that most of the titanium appeared in the form of extra-lattice titanium oxide and, lastly, that the zeolites obtained were not suitable for the catalysis of controlled oxidation reactions. This inadequacy of zeolites is also partly associated with the presence of acidic sites, especially with the presence of aluminium atoms in the silicon structure.
In order to limit, or even eliminate, the presence of extra-lattice titanium, i.e. titanium not inserted into the zeolite structure, and that of acidic sites, the present invention is aimed at obtaining titanium zeolites inserted into the lattice, by a simple and industrially exploitable treatment. This treatment is a post-synthesis treatment of zeolites with or without titanium, the titanium atoms being partly located at the surface or partly inserted into the structure, which makes it possible either to limit the synthesis steps or to enhance considerably the selectivity and activity of the catalyst comprising the zeolite with regard to controlled oxidation reactions. In addition, this post-synthesis treatment is directed towards obtaining titanium zeolites of greater stability and higher crystallinity, even in strongly acidic medium.